Help with slow leak

I have a slow leak in some sweated copper.
I'm replumbing part of my house - part of a new boiler install and hydronic system. I've started to test for leaks in my work, and on a complex part of the project, going from 1" -> 3/4" -> 1/2" with branches off each, totaling about 40' long, including the new water fill for the boiler, I've got a slow leak that I cannot find. I have a schrader valve with a pressure guage (up to 30 PSI) that I got at HD (I know - maybe that's the problem). Anyway, I pressurize up to 30, and I get a slow drift maybe about 1-2 PSI per hour. It drops over days to about 2-3 PSI and stops there. I've soaped every soldered joint, and a few PTFE-taped threaded joins in the boiler fill section, and I cannot find any bubbles forming. Is there another method for finding a leak? Do I need to use water, and look for drips? That seems like it might be more obvious. Help!

The air temp is probably changing. For example.....if your gauge reads 100psi and air temp at the time of the reading is 80 degrees.......if the air temp drops to 65 degrees the gauge would read less. if the air temp went up to 100 degrees the gauge would read more pressure. Simple as that.

The air temp is probably changing. For example.....if your gauge reads 100psi and air temp at the time of the reading is 80 degrees.......if the air temp drops to 65 degrees the gauge would read less. if the air temp went up to 100 degrees the gauge would read more pressure. Simple as that.

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Air temp won't account for it dropping to essentially zero...plus, with air temp, it would go both up and down with the temp, not consistently drop.

Don't know if this could be an issue, but some circulators use water for the bearing lubricant, and when dry, might (not sure) slowly leak a little there.

Terry - that's helpful, and in fact, when I first set it up, I did indeed find a soldered joint - one in an awkward position that I knew I was not as graceful making in the first place. It was audible and sputtering at 20-25 PSI. I took it apart, cleaned it up and resoldered, and its quiet now, and no bubble with soap either.
When that one was leaking, I would pressurize up to 25 or so and I could watch the needle drop to 0 in a minute or two - really fast.
Now that that is fixed, I pressurize up to 25 or 30, and the needle drops really slowly - it'll drop down to the teens overnight, and down to 2-3 in a day or two, and stop there. I hear no hissing anywhere, and like I say, I've soaked all the joints with soapy water and cannot find a source of bubbling.
There are two threaded joints - at either side of a Y-strainer at the boiler fill portion - but no bubbles. I was thinking already of taking this apart, re-taping and putting it back together.
The circulators in the boiler circuit are not seeing the pressure yet - I have the ball valves to the boiler from the cold water closed, and to the hop water pipes as well - I was trying to test in segments.

I'm going to take the test gauge down, re-tape it and see what happens. If that doesn't fix it, I'll take down the taped joints at the Y-strainer. And if that doesn't fix the loss of pressure, then I'm thinking that I'll charge the line with water and look for drips.

Go to a toy store and get some kid's bubble solution. Spray that on all the joints. If you have a slow leak, it will "cling" to the joint a lot longer and create bubbles if there is a leak. Plus, the bubbles will remain at the leak a lot longer than soap will.

Let me get this straight.
All of you came up with the suggestion to look at the gauge for leaks, instead of the rest of the house?
Are you serious. Do you think he didn't make sure the gauge had a tight connection?

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I've only done a few dozen air tests, but nearly all of them leaked at my cheap (HD) test setup. Also had a few other leaks at the temporary threaded joints at the ends, and one valve that leaked. I've never had a copper joint leak.

One time I had a stubborn gas leak that I couldn't find with soap until I upped the pressure to 80 psi (everything was capped, of course). Turned out to be defective threads in a tee.

Genius! HJ you were right! I used the kid's bubble juice - and found the leak - AND it was in the test equipment. I have a taped, threaded joint between the schrader valve/pressure gauge and the sharkbite fitting on the end of the pipe, and it's making bubbles like crazy.
I'll report back after I take it down an retape it. Damned HD.